In electrophotography an image comprising a pattern of electrostatic potential (also referred to as an electrostatic latent image), is formed on a surface of an electrophotographic element comprising at least an insulative photoconductive layer and an electrically conductive substrate. The electrostatic latent image is usually formed by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
In latent image formation the imagewise discharge is brought about by the radiation-induced generation of electron/hole pairs, by a material (often referred to as a charge-generation material) in the electrophotographic element. Depending upon the polarity of the initially uniform electrostatic potential and the type of materials in the electrophotographic element, either the holes or the electrons that have been generated migrate toward the charged surface in the exposed areas and cause the imagewise discharge of the initial potential. What remains is a non-uniform potential constituting the electrostatic latent image.
Many electrophotographic elements are designed to be initially charged with a negative polarity. They contain material, known as a hole-transport agent, which facilitates the migration of positive holes toward the negatively charged surface in imagewise exposed areas. A positively charged toner develops the unexposed areas. Because of the wide use of negatively charging elements, many types of positively charging toners are available. Conversely, relatively few high quality negatively charging toners are available.
For some applications, however, it is desirable to develop the exposed rather than the unexposed surface areas of the element. For example, in laser printing of alphanumeric characters it is more desirable to expose the small surface area that will form visible alphanumeric toner images, rather than waste energy exposing the large background area. In order to accomplish this with available high quality positively charging toners, it is necessary to use an electrophotographic element that is designed to be positively charged. Positive toner can then develop the exposed surface areas (which will have relatively negative electrostatic potential).
An electrophotographic element designed to be initially positively charged, however, preferably contains an electron-transport agent, i.e., a material which facilitates the migration of photogenerated electrons toward the positively charged surface. Unfortunately, many good hole-transport agents are available, but relatively few electron transport agents are known.
A number of chemical compounds having electron-transport properties are described, for example, in U.S. Pat. Nos. 4,175,960; 4,474,865; 4,559,287; 4,606,861; and 4,609,602. U.S. Pat. No. 4,514,481 discloses 4H-thiopyran-1,1-dioxide compounds as electron-transport agents and illustrates incorporating them in polymeric binder layers of electrophotographic elements. Unsymmetrically substituted 2,6-diaryl-4H-thiopyran-1,1-dioxide compounds are disclosed in U.S. Pat. Nos. 4,968,813, 5,013,849, 5,034,293, and 5,039,585.
Many electron-transport agents of the prior art have one or more drawbacks. Many do not perform the electron-transporting function well under certain conditions or in certain types of electrophotographic elements. Many agents require high exposure to discharge the surface potential of a charged electrophotographic element, resulting in low electrophotographic speed. Many agents also cause a high residual voltage to remain on the surface of the element after discharge, resulting in increased background density in the copy.
Thus, there is a continuing need for electrophotographic elements containing new chemical compounds that lower the amount of exposure required to discharge the surface potential of the charged elements, leading to increased electrophotographic speed. There is also a continuing need for electrophotographic elements containing compounds that lower the residual, or toe, voltage remaining on the surface after discharge. The present invention meets these needs.